System and method for measuring a sample by x-ray reflectance scatterometry

ABSTRACT

A system and method for measuring a sample by X-ray reflectance scatterometry. The method may include impinging an incident X-ray beam on a sample having a periodic structure to generate a scattered X-ray beam, the incident X-ray beam simultaneously providing a plurality of incident angles and a plurality of azimuthal angles; and collecting at least a portion of the scattered X-ray beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/686,953, filing date 18 Nov. 2019, (now U.S. Pat. No. 10,859,519,issued Dec. 8, 2020), which is a continuation of U.S. patent applicationSer. No. 16/181,287, filed on Nov. 5, 2018, (now U.S. Pat. No.10,481,112, issued Nov. 19, 2019), which is a continuation of U.S.patent application Ser. No. 15/451,104, filed on Mar. 6, 2017 (now U.S.Pat. No. 10,119,925, issued Nov. 6, 2018), which is a continuation ofU.S. patent application Ser. No. 14/161,942, filed on Jan. 23, 2014 (nowU.S. Pat. No. 9,588,066, filed Mar. 7, 2017), the entire contents ofwhich are hereby incorporated by reference herein.

BACKGROUND 1) Field

Embodiments of the invention are in the field of X-ray reflectancescatterometry (XRS) and, in particular, methods and systems formeasuring periodic structures using multi-angle XRS.

2) Description of Related Art

As integrated circuit (IC) features continue to be scaled to eversmaller dimensions, constraints on metrology used to measure suchfeatures become overwhelming. For example, critical dimension scanningelectron microscopy (CD-SEM) metrology has several drawbacks that arebecoming more significant with each new generation of IC technology.Drawbacks can include (1) the well known charging problem that limitsthe achievable resolution for IC metrology applications, (2) radiationdamage induced dimensional shrinking of resists, (3) incompatibilitywith some low-k dielectrics, and (4) CD-SEM is essentially a surfacetechnique making it difficult to measure three dimensional (3D)profiles.

Similarly, optical critical dimension (OCD) metrology faces a number offundamental difficulties, including (1) the relatively long wavelengthused is typically significantly larger than the device feature size andtherefore does not provide a simple and direct measurement, and (2) OCDrequires extensive modeling and interpolation, thus compromising themeasurement sensitivity. Furthermore, over the last decades, use ofshorter and shorter wavelengths has been necessitated by the reductionof circuit feature size. Currently the most advanced OCD system usesdeep ultraviolet (DUV) wavelengths. Further incremental reduction inwavelength is not practical because of the extremely low transmission ofshorter wavelength radiation in solids or even in low vacuum. Numerousproblems can arise as a consequence, including low probing depth, lackof suitable optics, and stringent vacuum requirements. Such fundamentallimitations have made it practically impossible to extend these existingtechnologies to meet the critical dimensional control requirements ofnext generation IC fabrication.

Grazing-incidence small-angle scattering (GISAS) is a scatteringtechnique used to study nanostructured surfaces and thin films. Thescattered probe is either photons (Grazing-incidence small-angle X-rayscattering, GISAXS) or neutrons (Grazing-incidence small-angle neutronscattering, GISANS). In either case, an incident beam strikes a sampleunder a small angle close to the critical angle of total external x-rayreflection. The intense reflected beam as well as the intense scatteringin the incident plane are attenuated by a rod-shaped beam stop. Thediffuse scattering from the sample is typically recorded with an areadetector. However, since incident angles used in GISAS techniques isusually less than a few degrees, and even as small as a fraction of adegree. Accordingly, when used to measure 3D structures, informationobtained through GISAS may be limited since the incident beam isdirected mostly along only the top surfaces of such 3D structures.

Thus, advances are needed in metrology of 3D structures.

SUMMARY

Embodiments of the present invention pertain to methods and systems formeasuring periodic structures using multi-angle X-ray reflectancescatterometry (XRS).

In an embodiment, a method of measuring a sample by X-ray reflectancescatterometry involves impinging an incident X-ray beam on a samplehaving a periodic structure to generate a scattered X-ray beam, theincident X-ray beam simultaneously providing a plurality of incidentangles and a plurality of azimuthal angles. The method also involvescollecting at least a portion of the scattered X-ray beam.

In another embodiment, a system for measuring a sample by X-rayreflectance scatterometry includes an X-ray source for generating anX-ray beam having an energy of approximately 1 keV or less. The systemalso includes a sample holder for positioning a sample having a periodicstructure. The system also includes a monochromator positioned betweenthe X-ray source and the sample holder. The monochromator is forfocusing the X-ray beam to provide an incident X-ray beam to the sampleholder. The incident X-ray beam simultaneously has a plurality ofincident angles and a plurality of azimuthal angles. The system alsoincludes a detector for collecting at least a portion of a scatteredX-ray beam from the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a periodic structuresubjected to conventional scatterometry measurements using an incidentbeam having a single angle of incidence.

FIG. 2 illustrates a cross-sectional view of a periodic structuresubjected to scatterometry measurements using an incident beam havingmultiple angles of incidence, in accordance with an embodiment of thepresent invention.

FIG. 3 illustrates a top-down view of a periodic structure subjected toconventional scatterometry measurements using an incident beam having asingle azimuthal angle.

FIG. 4A illustrates a top-down view of a periodic structure subjected toscatterometry measurements using an incident beam having multipleazimuthal angles, with a central axis having a zero azimuthal angle, inaccordance with an embodiment of the present invention.

FIG. 4B illustrates a top-down view of a periodic structure subjected toscatterometry measurements using an incident beam having multipleazimuthal angles, with a central axis having a non-zero azimuthal angle,in accordance with an embodiment of the present invention.

FIG. 5 illustrates aspects of exemplary fin-FET devices suitable for lowenergy X-ray reflectance scatterometry measurements, in accordance withan embodiment of the present invention.

FIG. 6 includes a plot and corresponding structures of 0th orderreflectance versus scattered angle silicon (Si) fins having a periodicstructure with 10 nm/20 nm line/space ratio, in accordance with anembodiment of the present invention.

FIG. 7 includes a plot and corresponding structures of 1st orderreflectance versus scattered angle silicon (Si) fins having a periodicstructure with 10 nm/20 nm line/space ratio, in accordance with anembodiment of the present invention.

FIG. 8 is an illustration representing a periodic structure measurementsystem having X-ray reflectance scatterometry (XRS) capability, inaccordance with an embodiment of the present invention.

FIG. 9 illustrates a block diagram of an exemplary computer system, inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Methods and systems for measuring periodic structures using multi-angleX-ray reflectance scatterometry (XRS) are described. In the followingdescription, numerous specific details are set forth, such as X-ray beamparameters and energies, in order to provide a thorough understanding ofembodiments of the present invention. It will be apparent to one skilledin the art that embodiments of the present invention may be practicedwithout these specific details. In other instances, well-known featuressuch as entire semiconductor device stacks are not described in detailin order to not unnecessarily obscure embodiments of the presentinvention. Furthermore, it is to be understood that the variousembodiments shown in the Figures are illustrative representations andare not necessarily drawn to scale.

One or more embodiments described herein are directed to the use of anX-ray source configured in a manner that exploits simultaneous multipleincoming beam angles incident on a periodic (grating) structure forX-ray reflectance scatterometry measurements. Embodiments may enabledetection of scattered light in two angular directions, as well as theuse of reflected X-ray intensities to infer the shape and pitch of aperiodic structure. Embodiments may provide suitable precision andstability measurements of the shape and size of complex two-dimensional(2D) and three-dimensional (3D) periodic structures in a production fabsemiconductor environment. Such measurements may include shape profileof the periodic structures, and dimensions such as width, height andside-wall angle of the periodic structures.

To provide context, state-of-the-art shape metrology solutions utilizeoptical techniques with either single-wavelength or spectral sourcesnominally greater than 150 nanometers in wavelength. Spectral solutionsare typically of fixed wavelength, and single wavelength sources thatcan vary in incident angle. Such solutions are in a wavelength/energyregime where λ>d, where λ is the incident light source, and d is thefundamental dimension of the periodic structure. However, opticalscatterometry is approaching its fundamental sensitivity limits.

In accordance with an embodiment of the present invention, by usingwavelengths of light where λ/d<1, higher order scattering orders areavailable for detection, and provide direct sensitivity to the parameterd. More specifically, by using wavelengths of light less than the widthand height of the structures being measured, interference fringes ofmultiple cycles are available, and provide sensitivity to height, widthand line shape. In an embodiment, by using multiple angles of incidenceas well as azimuthal angles (e.g., relative to the direction ofstructure symmetry), three-dimensional information is obtained,providing three-dimensional shape sensitivity. The information obtainedconcerns dimensions that can critically affect device performance, andneed to be controlled to very tight tolerances.

In order aid in conceptualizing concepts involved herein, FIG. 1illustrates a cross-sectional view of a periodic structure subjected toconventional scatterometry measurements using an incident beam having asingle angle of incidence. Referring to FIG. 1, a periodic structure 100(also referred to as a grating structure) is subjected to a light beam102. The light beam 102 has an angle of incidence, ϕi, relative to ahorizontal plane 104 of the uppermost surface of the periodic structure100. Scattered beams 106 are generated from the grating structure 100.The scattered beams 106 may include beams of differing scattered angle,each providing a different order of information of the grating structure100. For example, as shown in FIG. 1, three orders, n=1, n=0, n=−1, areshown, where the scattered angle for the n=−1 order has an angle of θrelative to the horizontal plane 104 of the uppermost surface of theperiodic structure 100. The arrangement of FIG. 1 is illustrative ofconventional OCD or GISAS scatterometry approaches.

It is to be appreciated that use of the terms “periodic” or “grating”structure throughout refers to structures that are non-planar and, insome contexts, can all be viewed as three-dimensional structures. Forexample, referring again to FIG. 1, the periodic structure 100 hasfeatures 108 that protrude in the z-direction by a height, h. Eachfeature 108 also has a width, w, along the x-axis and a length along they-axis (i.e., into the page). In some contexts, however, the term“three-dimensional” is reserved to describe a periodic or gratingstructure having a length along the y-axis that is on the same order asthe width, w. In such contexts, the term “two-dimensional” is reservedto describe a periodic or grating structure having a length along they-axis that is substantially longer than the width, w, e.g., severalorders of magnitude longer. In any case, a periodic or grating structureis one having a non-planar topography within a region of measurement of,e.g., a semiconductor wafer or substrate.

In contrast to FIG. 1, FIG. 2 illustrates a cross-sectional view of aperiodic structure subjected to scatterometry measurements using anincident beam having multiple angles of incidence, in accordance with anembodiment of the present invention. Referring to FIG. 2, the periodicstructure 100 is subjected to a conical X-ray beam 202. The conicalX-ray beam 202 has a central axis 203 having an angle of incidence, ϕi,relative to the horizontal plane 104 of the uppermost surface of theperiodic structure 100. As such, the conical X-ray beam 202 includes aportion, A, that has an incident angle ϕi. The conical beam 202 has aconverging angle, ϕcone, which is taken between outermost portion, B,and outermost portion, C, of the conical beam 202. Since the conicalbeam 202 has the converging angle ϕcone, portions of the conical beam202 near the outer portion of the cone have a different angle ofincidence on the structure 100 than the portions of the conical beam 202that are aligned with the central axis 202. Accordingly, the conicalbeam 202 simultaneously provides multiple angles of incidence forimpinging on the structure 100, as taken relative to the horizontalplane 104. A scattered beam 206 is generated from the grating structure100. The scattered beam 206 may include portions attributable todifferent orders of information of the grating structure 100, examplesof which are described in greater detail below.

In addition to having an angle of incidence, an incident light beam canalso have an azimuthal angle with respect to a periodic structure. Againfor conceptual purposes, FIG. 3 illustrates a top-down view of aperiodic structure subjected to conventional scatterometry measurementsusing an incident beam having a single azimuthal angle. Referring toFIG. 3, the periodic structure 100 is shown from above the protrudingportions 108. Although not viewable in FIG. 1, the incident light beam102 can further have an azimuthal angle, θg, relative to a direction, x,which is orthogonal to the protrusions 108 of the periodic structure100. In some cases, θg is non-zero, as is depicted in FIG. 3. In caseswhere θg is zero, the direction of the beam 102 is along thex-direction, with respect to the top-down view. In all cases whereconventional OCD or GISAS scatterometry approaches applied, however, thebeam 102 has only one angle, θg. Thus, taking FIGS. 1 and 3 together,conventionally, scatterometry is performed using a light beam having asingle angle of incidence, ϕi, and a single azimuthal angle, θg.

In contrast to FIG. 3, FIGS. 4A and 4B illustrate top-down views of aperiodic structure subjected to scatterometry measurements using anincident beam having multiple azimuthal angles, in accordance with anembodiment of the present invention. Referring to both FIGS. 4A and 4B,the periodic structure 100 is subjected to the conical X-ray beam 202having the central axis 203, as described in association with FIG. 2.Although not viewable from FIG. 2, the conical X-ray beam 202 furtherhas dimensionality along the y-direction. That is, the converging angle,ϕcone, taken between outermost portion, B, and outermost portion, C, ofthe conical beam 202, also provides a plurality of incident angles alongthe y-direction, e.g., to provide non-zero azimuthal angles ofincidence.

Referring only to FIG. 4A, the central axis of the conical X-ray beam202 has an angle θg of zero along the x-direction, with respect to thetop-down view. As such, the portion A the conical X-ray beam 202 has azero azimuthal angle. Nonetheless, the portions B and C of the conicalX-ray beam 202 have non-zero azimuthal angles even though the centralaxis 203 of the conical X-ray beam 202 is orthogonal to the periodicstructure 100.

Referring only to FIG. 4B, the central axis of the conical X-ray beam202 has a non-zero angle, θg, along the x-direction, with respect to thetop-down view. As such, the portion A the conical X-ray beam 202 has anon-zero azimuthal angle. Additionally, the portions B and C of theconical X-ray beam 202 have non-zero azimuthal angles different from theazimuthal angle of portion A of the beam 202.

In both cases illustrated in FIG. 4A and FIG. 4B, since the conical beam202 has the converging angle ϕcone, portions of the conical beam 202near the outer portion of the cone have a different azimuthal angleincident on the structure 100 than the portions of the conical beam 202that are aligned with the central axis 202. Accordingly, the conicalbeam 202 simultaneously provides multiple azimuthal angles for impingingon the structure 100, as taken relative to the x-direction.

Thus, taking FIG. 2 and one of FIG. 4A or 4B together, in accordancewith an embodiment of the present invention, a method of measuring asample by X-ray reflectance scatterometry involves impinging an incidentX-ray beam on a sample having a periodic structure. The X-ray beam has aconical shape to simultaneously provide multiple angles of incidence,ϕi, and multiple azimuthal angles, θg, as incident on the periodicstructure. The impinging generates a scattered X-ray beam, a portion ofwhich (if not all) can be collected in order to glean information aboutthe periodic structure.

In an embodiment, the incident X-ray beam is a converging X-ray beamhaving a converging angle, ϕcone, approximately in the range of 20-40degrees. In one such embodiment, a central axis of the converging X-raybeam has a fixed non-zero incident angle, ϕi, and an azimuthal angle,θg, of zero relative to the sample, as was described in association withFIG. 4A. In another such embodiment, a central axis of the convergingX-ray beam has a fixed non-zero incident angle, ϕi, and a non-zeroazimuthal angle, θg, relative to the sample, as was described inassociation with FIG. 4B. In either case, in a specific embodiment, thecentral axis of the converging X-ray beam has the fixed non-zeroincident angle approximately in the range of 10-15 degrees fromhorizontal. In another specific embodiment, the outermost portion of theconical shape of the beam and closest portion to the periodic structure,e.g., portion C as shown in FIG. 2, has an angle of approximately 5degrees relative to a horizontal plane of the periodic structure.

In other embodiments, an example of which is described in greater detailbelow, it may be preferable to use a narrower conical shape. Forexample, in an embodiment, the incident X-ray beam is a converging X-raybeam having a converging angle approximately in the range of 2-10degrees. In one such embodiment, a central axis of the converging X-raybeam has a fixed non-zero incident angle, ϕi, and an azimuthal angle,θg, of zero relative to the sample, as was described in association withFIG. 4A. In another such embodiment, a central axis of the convergingX-ray beam has a fixed non-zero incident angle, ϕi, and a non-zeroazimuthal angle, θg, relative to the sample, as was described inassociation with FIG. 4B.

In an embodiment, a low energy X-ray beam is impinged on the periodicstructure. For example, in one such embodiment, the low energy X-raybeam has an energy of approximately 1 keV or less. Use of such a lowenergy source can allow for larger incident angles yet with a smallerachievable spot size. In one embodiment, the low energy X-ray beam is aKα beam generated from a source such as, but not limited to, carbon (C),molybdenum (Mo) or Rhodium (Rh).

In an embodiment, the low energy X-ray beam is focused using a toroidalmultilayer monochromator prior to impinging on the periodic structure.In one such embodiment, the monochromator provides an incident anglerange of approximately +/−30 degrees and an azimuth angle range ofapproximately +/−10 degrees. In a specific such embodiment, the toroidalmultilayer monochromator provides an incident angle range ofapproximately +/−20 degrees. It is to be appreciated that the conicalX-ray beams described herein may not, or need not, be collimated. Forexample, in one embodiment, between focusing the beam at the abovedescribed monochromator and impinging the focused beam on the periodicsample, the beam is not subjected to collimation. In one embodiment, thefocused low energy X-ray beam is impinged on the sample at an incidentangle range less than the angle of a nominal first-order angle at zerodegrees.

Referring again to FIG. 2, in an embodiment, at least a portion of thescattered X-ray beam 206 is collected using a detector 250. In one suchembodiment, a two-dimensional detector is used to simultaneously samplescattered signal intensity of the portion of the scattered X-ray beam206 scattered from the plurality of incident angles and the plurality ofazimuthal angles. The collected signal may then be subjected toscatterometry analysis, e.g., where inversion of scatter data iscompared to theory to determine structural details of the periodicstructure 100. In one such embodiment, a shape of the periodic structureof a sample is estimated by inversion of scattering solutions relativeto the sampled scattered signal intensity, e.g., by rigorously solvingMaxwell's equations on the periodic structure. In an embodiment, theX-ray beam impinged on the sample has a wavelength less than aperiodicity of the periodic structure 100. Thus, the probing wavelengthis comparable to or less than fundamental structural dimensions,providing a richer set of data from the scattered beam 206 as comparedto OCD scatterometry.

As described above, in an embodiment, the incident conical X-ray beamused for XRS is a converging X-ray beam having a converging angle,ϕcone, approximately in the range of 20-40 degrees. Such a relativelybroad cone angle may generate a scattered beam that includes higherorder diffraction data in addition to zero-order reflection data. Thus,in one embodiment, both zero order and higher order information areobtained in parallel with a single impinging operation.

In other scenarios, it may be desirable to separate zero orderreflection data from higher order diffraction data. In one suchembodiment, a relatively narrower cone angle may be used, e.g., theincident X-ray beam is a converging X-ray beam having a converging angleapproximately in the range of 2-10 degrees. More than one singlemeasurement may be performed using the relatively narrower cone angle.For example, in one embodiment, a first measurement is made where thecentral axis of the converging beam has an azimuthal angle of zero, asdescribed in association with FIG. 4A. A second measurement is then madewhere the central axis of the converging beam has a non-zero azimuthalangle, as described in association with FIG. 4B. In a specificembodiment, in a sequential manner, the first measurement is performedto collect 0th order but not 1st order diffraction data for a samplehaving a periodic structure. The second measurement is performed tocollect 1st order but not 0th order diffraction data for a sample havinga periodic structure. In this way, zero order data can be separated fromhigher order data at the time of generating the scattered beam.

Pertaining again to both the parallel and sequential approaches, inaccordance with embodiments described herein, X-ray reflectancescatterometry is used to separate different orders on an array detectorby approaching in a non-zero azimuth. In many cases it is the higherorders that are more useful. By cleanly obtaining all the orders inparallel, in one case, throughput can be enhanced. However, sequentialapproaches may also be used. Furthermore, a very focused beam is used toprobe at a variety of incidence angles rather than at a single angle ofincidence. In one embodiment, the beam is not collimated since for acollimated beam, a sample would require rotation with data takenserially. By capturing a higher order, use of a very small incidenceangle is not needed in order to obtain a strong reflected beam. Bycontrast, in an embodiment, an angle of incidence of, e.g., 10 degreesto 15 degrees can be used even in the case where a specular (0-order)reflected beam is relatively weak but the −1 order, for example, is verystrong.

In either case described above, whether collected in parallel orsequentially, embodiments described herein can be used to acquire datafrom both the zero order (specular) reflection and from the diffracted(higher) orders. Conventional solutions have emphasized using eitherzero order or diffracted (higher) orders, but not both. Embodimentsdescribed herein can further be distinguished from prior disclosedscatterometry approaches, a couple examples of which are describedbelow.

In a first previously described approach, U.S. Pat. No. 7,920,676 to Yunet al. describes a CD-GISAXS system and method. The described approachinvolves analyzing the diffraction pattern of scattered X-rays generatedfrom a collimated beam and analyzing multiple orders of the diffractedlight. Lower energy is used to provide a higher-convergence beam becausethe diffraction orders are spaced farther apart. However, the orders arestill fairly closely spaced and the convergence angles described are inmicro-radians. Furthermore, diffraction is not collected for a multitudeof incidence angles.

By contrast, in accordance with one or more embodiments describedherein, a wide range of incidence angles is used in a single beam. Inthe present approach, diffracted orders (other than zero-order) do notactually have to be captured to be useful. However, the +/−1 orders canhave different sensitivities to grating characteristics (in particular,the pitch), so, in one embodiment, at least one extra order is capturedwhen possible. Even so, the bulk of the information is contained in theway the signal varies with incident angle. By contrast, in the U.S. Pat.No. 7,920,676, essentially one incident angle is used and information isgathered by looking at a multiplicity of diffracted orders.

Furthermore, in accordance with one or more embodiments describedherein, the first order beam can be separated from the zero-order beamby moving the first-order beam to the side of the zero-order beam. Inone such embodiment, the periodic or grating structure is approached ata non-zero azimuthal angle. In this way, a highly converging beam can beused while still achieving order separation. In an exemplary embodiment,by approaching the grating at a 45° azimuth angle (for the central axisof the converging beam), the +/−1 order diffracted beams are deflectedto the side of the zero-order beam by a minimum of 10 degrees, and evenmore as the incidence angle is increased. In this case, a convergentbeam of up to approximately 10 degrees can be used while avoidingoverlap or data. It is to be appreciated that depending on the specificsof the grating pitch and the X-ray energy, the separation between orderscan be made to be larger or smaller. Overall, in an embodiment, bycollecting a multiplicity of incident and azimuthal anglessimultaneously, more useful information is obtained than compared to asingle shot of a collimated beam.

In a second previously described approach, U.S. Pat. No. 6,556,652 toMazor et al. describes measurement of critical dimensions using X-rays.The described approach is not actually based on the diffraction of anX-ray beam at all. Instead, a “shadow” is created in a collimated beam.The shadow reflects off of a pattern (e.g., a linear grating structure).The contrast mechanism for the shadow is the difference in the criticalangle for reflecting x-rays between a Si region at the bottom of agrating gap and the critical angle when passing first through ridgematerial (photoresist). By contrast, in accordance with embodimentsdescribed herein, a majority of information comes from signals at anglesfar above the critical angle.

As mentioned briefly above, and exemplified below, X-ray reflectancescatterometry (XRS) can be viewed as a type of X-ray reflectometry (XRR)as applied to two-dimensional and three-dimensional periodic or gratingstructures. Traditional XRR measurements involve the use of a singlesource X-ray that probes a sample over a range of angles. Varyingoptical path length differences with angle provides interference fringesthat can be discerned to glean film property information such as filmthickness and film density. However, in XRR, physics of the X-rayinteraction with matter at higher source energies limits the angularrange to a grazing incidence of typically less than approximately threedegrees relative to sample horizontal plane. As a result, XRR has hadlimited production/inline viability. By contrast, in accordance withembodiments described herein, application of low-energy XRR/XRS enablesthe use of larger angles due to changing optical film properties withenergy that lead to larger angles of signal sensitivity.

In an exemplary application of low energy XRS, fundamental semiconductortransistor building blocks may be measured and analyzed. For example, acritical dimension (CD) of a semiconductor device refers to a featurethat has a direct impact on device performance or its manufacturingyield. Therefore, CDs must be manufactured or controlled to tightspecifications. Examples of more conventional CDs include gate length,gate width, interconnect line width, line spacing, and line widthroughness (LWR). Semiconductor devices are very sensitive to suchdimensions, with small variations potentially leading to substantialimpacts on performance, device failure, or manufacturing yield. Asintegrated circuit (IC) feature sizes of semiconductor devices continueto shrink, manufacturers face ever decreasing process windows andtighter tolerances. This has dramatically raised the accuracy andsensitivity requirements for CD metrology tools as well as the need fornon-destructive measurement sampling early in the manufacturing cyclewith minimal impact to productivity of the semiconductor devicemanufacturing plant or fab.

Non-planar semiconductor device fabrication has complicated matters evenfurther. For example, semiconductor devices fabricated on raisedchannels having a non-planar topography often referred to as finsfurther include fin dimensions as additional CDs that must be accountedfor. Such fin field effect transistor (fin-FET) or multi-gate deviceshave high-aspect ratio features, and the need for three-dimensional (3D)profile information on the fins of device structures, including sidewallangle, and top and bottom dimensions, has become critical. Consequently,the ability to measure the 3D profile provides far more valuableinformation than the conventional two-dimensional line width and spacingCD information.

FIG. 5 illustrates aspects of exemplary fin-FET devices suitable for lowenergy X-ray reflectance scatterometry measurements, in accordance withan embodiment of the present invention. Referring to FIG. 5, structure Aillustrates an angled cross-sectional view of a semiconductor fin 502having a gate electrode stack 504 disposed thereon. The semiconductorfin 502 protrudes from a substrate 506 which is isolated by shallowtrench isolation (STI) regions 508. The gate electrode stack 504includes a gate dielectric layer 510 and a gate electrode 512. StructureB illustrates a cross-sectional view of a semiconductor fin 520protruding from a substrate 522 between STI regions 524. Aspects ofstructure B that may provide important information through XRSmeasurements include fin corner rounding (CR), fin sidewall angle (SWA),fin height (H), fin notching (notch), and STI thickness (T), all ofwhich are depicted in structure B of FIG. 5. Structure C illustrates across-sectional view of a semiconductor fin 530 protruding from asubstrate 532 between STI regions 534, and having a multilayer stack offilms 536 thereon. The layers of the multilayer stack of films 536 mayinclude material layers such as, but not limited to, titanium aluminumcarbide (TiAlC), tantalum nitride (TaN) or titanium nitride (TiN).Comparing structures B and C, XRS measurements may be performed on abare fin such as a bare silicon fin (structure B), or on a fin havingdifferent material layers disposed thereon.

FIG. 6 includes a plot 600 and corresponding structures (A)-(E) of 0thorder reflectance versus scattered angle silicon (Si) fins having aperiodic structure with 10 nm/20 nm line/space ratio, in accordance withan embodiment of the present invention. Referring to FIG. 6, low energyXRS measurements can be used to distinguish between a nominal finstructure (structure A), a structure of increased fin height (structureB), a structure of decreased fin width (structure C), a structure ofwider fin bottom CD versus fin top CD (structure D), and a structure ofnarrower fin bottom CD versus fin top CD (structure E). In this exemplarcase, the Si fins are analyzed with 0th order conical diffraction at 45degrees to the periodic structure. It is to be appreciated that, incomparison to optical data, a reduced region of highest signal isachieved with fringes in data seen in plot 600 being a consequence ofshort wavelength.

FIG. 7 includes a plot 700 and corresponding structures (A)-(E) of 1storder reflectance versus scattered angle silicon (Si) fins having aperiodic structure with 10 nm/20 nm line/space ratio, in accordance withan embodiment of the present invention. Referring to FIG. 7, low energyXRS measurements can be used to distinguish between a nominal finstructure (structure A), a structure of increased fin height (structureB), a structure of decreased fin width (structure C), a structure ofwider fin bottom CD versus fin top CD (structure D), and a structure ofnarrower fin bottom CD versus fin top CD (structure E). In this exemplarcase, the Si fins are analyzed with 1st order conical diffraction at 45degrees to the periodic structure. Additionally, a structure of varyingpitch has been included in plot 700. As shown in plot 700, 1st orderdata is very sensitive to fin thickness (noting that structure B isseparated significantly from the signals due to structures A and C-E).Also, 1st order data is very sensitive to pitch variation in theperiodic structure, noting that the spectrum for varied pitch is alsosignificantly discernible from the other spectra.

In another aspect, an apparatus for performing X-ray reflectancescatterometry is described. In general, in an embodiment, such anapparatus includes a generic X-ray source along with a focusingmonochromator that extends in two dimensions. The focusing monochromatorallows for incident rays of light to strike a periodic sample at twovarying incident angles, (i) incident to the plane of the periodicstructure, and (ii) azimuthally, with respect to the symmetry of thestructure (and at fixed incident angle). The detection of the scatteredlight is achieved by a two-dimensional (2D) detector, whichsimultaneously samples the scattered signal intensity across the rangeof scattered angles in the two angular directions. In one embodiment,the constraints of the monochromator that assure the detected signal isfree of scattering order-overlap require that the incident angle rangebe less than the angle of the nominal first-order angle at θ degree,i.e., θ=sin−1 (1−λ/d). As a result of the use of light with acharacteristic wavelength smaller than the period of the grating, higherorder diffraction orders are accessible, and provide additionalinformation regarding the grating structure. In addition, interferencefringes of multiple thickness cycles are available to determine lineheight, width and shape. The final estimation of the shape and structureof the periodic structure is achieved via inversion of the scatteringsolutions compared to the 2D interference/scatter data.

As a more specific example, FIG. 8 is an illustration representing aperiodic structure measurement system having XRS capability, inaccordance with an embodiment of the present invention.

Referring to FIG. 8, a system 800 for measuring a sample 802 by X-rayreflectance scatterometry includes an X-ray source 804 for generating anX-ray beam 806 having an energy of approximately 1 keV or less. A sampleholder 808 is provided for positioning the sample 802, the sample havinga periodic structure. A monochromator 810 is positioned between theX-ray source 804 and the sample holder 802, in that the X-ray beam 806travels from the X-ray source 804 to the monochromator 810 and then tothe sample holder 808. The monochromator 810 is for focusing the X-raybeam 806 to provide an incident X-ray beam 812 to the sample holder 808.The incident X-ray beam 812 simultaneously has a plurality of incidentangles and a plurality of azimuthal angles. The system 800 also includesa detector 814 for collecting at least a portion of a scattered X-raybeam 816 from the sample 802.

Referring again to FIG. 8, in an embodiment, the X-ray source 804, thesample holder 808, the monochromator 810 and the detector 814 are allhoused in a chamber 818. In an embodiment, the system 800 furtherincludes an electron gun 820. In one such embodiment, the X-ray source804 is an anode and the electron gun is directed at the anode. In aparticular embodiment, the anode is for generating low energy X-rays andincludes a material such as, but not limited to, carbon (C), molybdenum(Mo) or Rhodium (Rh). In one embodiment, the electron gun 820 is anapproximately 1 keV electron gun. Referring again to FIG. 8, a magneticelectron suppression device 822 is included between the X-ray source 804and the monochromator 810.

In an embodiment, the monochromator 810 is a toroidal multilayermonochromator that provides an incident angle range of approximately+/−30 degrees and an azimuth angle range of approximately +/−10 degrees.In one such embodiment, the toroidal multilayer monochromator providesan incident angle range of approximately +/−20 degrees. In anembodiment, as described above, there is no intervening collimatorbetween the monochromator 810 and the sample holder 808. Themonochromator 810 may be positioned to provide a desired incident beamfor XRS measurements. For example, in a first embodiment, themonochromator 810 is positioned relative to the sample holder 808 toprovide a converging X-ray beam having a central axis with a fixednon-zero incident angle and an azimuthal angle of zero relative to aperiodic structure of a sample 802. In a second embodiment, themonochromator 810 is positioned relative to the sample holder 808 toprovide a converging X-ray beam having a central axis with a fixednon-zero incident angle and a non-zero azimuthal angle relative to aperiodic structure of a sample 802. In an embodiment, the monochromator810 is composed of alternating metal (M) layers and carbon (C) layersdisposed on a glass substrate, where M is a metal such as, but notlimited to, cobalt (Co) or chromium (Cr). In a particular suchembodiment, a multilayer monochromator is provided for reflecting carbon(C) based Kα radiation and includes approximately would be 100 repeatinglayers of Co/C or Cr/C with a period of about 4 nanometers, i.e., aperiod slightly less than the wavelength of the reflected beam which maybe approximately 5 nanometers. In one such embodiment, the Co or Crlayers are thinner than the C layers.

The sample holder 808 may be a moveable sample holder. For example, inan embodiment, the sample holder 808 is rotatable to change an azimuthangle of a central axis of the X-ray beam 812 relative to a periodicstructure of a sample 802. In an embodiment, the sample holder 808 isrotatable to provide orthogonal operation with eucentric rotation,enabling two or more sample rotations per measurement. In an embodiment,a navigation visual inspection apparatus 824 allows visual inspection ofthe sample holder 808, as is depicted in FIG. 8. In one such embodiment,a flip-in objective lens is included for a vision-based inspectionsystem.

In an embodiment, the detector 814 is a two-dimensional detector. Thetwo-dimensional detector may be configured for simultaneously samplingscattered signal intensity of the portion of the scattered X-ray beam816 scattered from the plurality of incident angles and the plurality ofazimuthal angles of the incident beam 812. In an embodiment, the system800 further includes a processor or computing system 899 coupled to thetwo-dimensional detector. In one such embodiment, the processor 899 isfor estimating a shape of the periodic structure of a sample 802 byinversion of scattering solutions relative to the sampled scatteredsignal intensity. In place of a two-dimensional detector, in anotherembodiment, a scanning slit may be implemented. In either case, thedetector 814 can be configured to achieve approximately 1000 pixels ofdata collection across a dispersion range.

Embodiments of the present invention may be provided as a computerprogram product, or software, that may include a machine-readable mediumhaving stored thereon instructions, which may be used to program acomputer system (or other electronic devices) to perform a processaccording to the present invention. A machine-readable medium includesany mechanism for storing or transmitting information in a form readableby a machine (e.g., a computer). For example, a machine-readable (e.g.,computer-readable) medium includes a machine (e.g., a computer) readablestorage medium (e.g., read only memory (“ROM”), random access memory(“RAM”), magnetic disk storage media, optical storage media, flashmemory devices, etc.), a machine (e.g., computer) readable transmissionmedium (electrical, optical, acoustical or other form of propagatedsignals (e.g., infrared signals, digital signals, etc.)), etc.

FIG. 9 illustrates a diagrammatic representation of a machine in theexemplary form of a computer system 900 within which a set ofinstructions, for causing the machine to perform any one or more of themethodologies discussed herein, may be executed. In alternativeembodiments, the machine may be connected (e.g., networked) to othermachines in a Local Area Network (LAN), an intranet, an extranet, or theInternet. The machine may operate in the capacity of a server or aclient machine in a client-server network environment, or as a peermachine in a peer-to-peer (or distributed) network environment. Themachine may be a personal computer (PC), a tablet PC, a set-top box(STB), a Personal Digital Assistant (PDA), a cellular telephone, a webappliance, a server, a network router, switch or bridge, or any machinecapable of executing a set of instructions (sequential or otherwise)that specify actions to be taken by that machine. Further, while only asingle machine is illustrated, the term “machine” shall also be taken toinclude any collection of machines (e.g., computers) that individuallyor jointly execute a set (or multiple sets) of instructions to performany one or more of the methodologies discussed herein. For example, inan embodiment, a machine is configured to execute one or more sets ofinstruction for measuring a sample by X-ray reflectance scatterometry.In one example, the computer system 900 may be suitable for use ofcomputer system 899 of the above described XRS apparatus 800.

The exemplary computer system 900 includes a processor 902, a mainmemory 904 (e.g., read-only memory (ROM), flash memory, dynamic randomaccess memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM(RDRAM), etc.), a static memory 906 (e.g., flash memory, static randomaccess memory (SRAM), etc.), and a secondary memory 918 (e.g., a datastorage device), which communicate with each other via a bus 930.

Processor 902 represents one or more general-purpose processing devicessuch as a microprocessor, central processing unit, or the like. Moreparticularly, the processor 902 may be a complex instruction setcomputing (CISC) microprocessor, reduced instruction set computing(RISC) microprocessor, very long instruction word (VLIW) microprocessor,processor implementing other instruction sets, or processorsimplementing a combination of instruction sets. Processor 902 may alsobe one or more special-purpose processing devices such as an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA), a digital signal processor (DSP), network processor, or thelike. Processor 902 is configured to execute the processing logic 926for performing the operations discussed herein.

The computer system 900 may further include a network interface device908. The computer system 900 also may include a video display unit 910(e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), analphanumeric input device 912 (e.g., a keyboard), a cursor controldevice 914 (e.g., a mouse), and a signal generation device 916 (e.g., aspeaker).

The secondary memory 918 may include a machine-accessible storage medium(or more specifically a computer-readable storage medium) 931 on whichis stored one or more sets of instructions (e.g., software 922)embodying any one or more of the methodologies or functions describedherein. The software 922 may also reside, completely or at leastpartially, within the main memory 904 and/or within the processor 902during execution thereof by the computer system 900, the main memory 904and the processor 902 also constituting machine-readable storage media.The software 922 may further be transmitted or received over a network920 via the network interface device 908.

While the machine-accessible storage medium 931 is shown in an exemplaryembodiment to be a single medium, the term “machine-readable storagemedium” should be taken to include a single medium or multiple media(e.g., a centralized or distributed database, and/or associated cachesand servers) that store the one or more sets of instructions. The term“machine-readable storage medium” shall also be taken to include anymedium that is capable of storing or encoding a set of instructions forexecution by the machine and that cause the machine to perform any oneor more of the methodologies of the present invention. The term“machine-readable storage medium” shall accordingly be taken to include,but not be limited to, solid-state memories, and optical and magneticmedia.

In accordance with an embodiment of the present invention, anon-transitory machine-accessible storage medium has stored thereoninstruction for performing a method of measuring a sample by X-rayreflectance scatterometry. The method involves impinging an incidentX-ray beam on a sample having a periodic structure to generate ascattered X-ray beam. The incident X-ray beam simultaneously provides aplurality of incident angles and a plurality of azimuthal angles. Themethod also involves collecting at least a portion of the scatteredX-ray beam.

Thus, methods and systems for measuring periodic structures usingmulti-angle X-ray reflectance scatterometry (XRS) have been described.

What is claimed is:
 1. A method of measuring a sample by X-rayreflectance scatterometry, the method comprising: impinging an incidentX-ray beam on a sample having a periodic structure to generate ascattered X-ray beam, the incident X-ray beam simultaneously providing aplurality of incident angles and a plurality of azimuthal angles; andcollecting at least a portion of the scattered X-ray beam.
 2. A systemfor measuring a sample by X-ray reflectance scatterometry, the systemcomprising: an X-ray source for generating an X-ray beam having anenergy of approximately 1 keV or less; a sample holder for positioning asample having a periodic structure; a monochromator positioned betweenthe X-ray source and the sample holder, the monochromator for focusingthe X-ray beam to provide an incident X-ray beam to the sample holder,the incident X-ray beam simultaneously having a plurality of incidentangles and a plurality of azimuthal angles; and a detector forcollecting at least a portion of a scattered X-ray beam from the sample.